Capacitive micromachined ultrasonic transducer (“cMUT”) devices generally combine mechanical and electronic components in very small packages. The mechanical and electronic components operate together to transform mechanical energy into electrical energy and vice versa. Because cMUTs are typically very small and have both mechanical and electrical parts, they are commonly referred to as micro-electronic mechanical systems (“MEMS”) devices.
Conventional cMUTs generally have a ground electrode and a hot electrode. The hot electrode can be used to transmit and receive ultrasonic acoustical waves during ultrasonic imaging. Due to the differing characteristics associated with transmitting and receiving ultrasonic waves, conventional cMUT hot electrodes are commonly optimized to receive or transmit ultrasonic waves with high sensitivity while the maximum transmit power is compromised. This optimization results in sacrificed device performance for the non-optimized action, ultimately sacrificing the quality of data sent or collected by a cMUT having a hot electrode only optimized for receiving or transmit waves.
One approach to address this problem includes using two cMUTs placed side by side. One of the cMUTs can be optimized for transmitting ultrasonic waves while the other cMUT can be optimized for receiving ultrasonic waves. This approach, while addressing the problems associated with a hot electrode optimized for transmitting or receiving ultrasonic waves, has several drawbacks. For example, a major drawback associated with this solution includes the amount of space sacrificed when two cMUTs are used to transmit and receive ultrasonic waves instead of one. cMUTs are often desirable for small scale applications and the increased space associated with using multiple cMUTs would greatly diminish the desirability of using cMUTs.
Another problem associated with conventional cMUTs is that they have lower maximum transmit pressures relative to piezoelectric ultrasonic transducers. Higher transmit pressures are generally desirable to increase signal penetration and to increase the force of ultrasonic waves reflected from media being imaged. In conventional cMUT operation, where the membrane is moved from a rest position to near collapse, the output pressure is limited because the displacement is limited to the one third of the initial gap. To increase the output transmission pressure output for cMUTs to, for example, 1 MPa (Mega-Pascal), which is desired for medical imaging applications, collapse and collapse-snap back operation states for cMUT membranes have been proposed. These proposed approaches, while increasing cMUT output transmit pressures, are nonlinear operational modes limiting the transmit signal shapes, and can cause severe dielectric charging because of the frequent contact between dielectric layers during operation.
Another problem associated with conventional cMUTs is the difficulty in controlling the shape of a cMUT membrane. Since the geometrical shape of a cMUT membrane largely affects overall cMUT device performance, it is desirable to control a cMUT membrane's shape when transmitting and receiving ultrasonic waves.
An additional disadvantage to conventional cMUTs includes the complicated switching circuitry and signal generation and detection circuitry that must be used in concert with the single receive and transmit cMUT electrode. Typically, complicated circuitry is required for single receive and transmit cMUT electrode because protection circuits must be utilized to prevent large transmit signals to be input to the receiver amplifier. These protection circuits increase the parasitic capacitance in parallel with the cMUT, further degrading the received signal. The switching transients may saturate the receiving amplifier and result in dead zones in the images where the regions close to the transducer array can not be imaged.
Therefore, there is a need in the art for a cMUT fabrication method enabling fabrication of a cMUT with at least one multiple-element electrode to increase and enhance cMUT device performance.
Additionally, there is a need in the art for fabricating cMUTs having controllable membranes to increase and enhance cMUT device performance.
Additionally, there is a need in the art for a cMUT device having multiple hot electrodes or separate transmit and receive electrodes so transmitting and receiving ultrasonic wave functions can be simultaneously optimized.
Still yet, there is a need for cMUTs capable of generating multiple output signals from a single transmit event and utilizing the different frequency content of output signals.
It is to the provision of such cMUT fabrication and cMUT imaging array fabrication that the embodiments of present invention are primarily directed.